Page 40 - Physics 10_Float

GEOMETRICAL OPTICS

surfaces. Fig.12.6 shows how light is reflected by the Spoon as mirror
spherical surfaces of concave and convex mirrors according
to the two laws of reflection.
Normal Reflected Reflected ray
N angle Concave mirror
Incident
angle r
Incident ray i r i
Convex mirror i = r A well polished spoon acts as
i = r N

(left) mirrors.
Fig.12.6: Reflection of light by spherical mirrors convex (right) and concave
Physics insight
Activity12.2: Take a convex mirror or a well polished spoon
(using the outside of the spoon, with the convex surface Viewer Radius
bulging outward), and hold it in one hand. Hold a pencil with C
its tip in the upright position in the other hand. Try to look at Centre of
curvature
its image in the mirror. Is the image erect or inverted? Is the Pole
Principal axis
image smaller or larger in size than the object? Move the For a convex mirror, focus and
pencil away from the mirror. Does the image become smaller centre of curvature lie behind
or larger? Guess, whether the image will move closer to or the mirror.
farther from the focus? Point to ponder

12.3 IMAGE LOCATION BY SPHERICAL MIRROR
FORMULA
How can we tell about the nature of image (whether image is real
or imaginary, inverted or erect) formed in a mirror? How can we
tell about the size of the image compared with the size of the
object? To answer these questions, one method is graphical or In large shopping centres, convex
mirrors are used for security
ray diagram. But, we can also answer these questions by using a purposes. Do you know why?
mathematical formula called the mirror formula defined as:
For your information
The focal length of a spherical
Mirror formula is the relationship between object distance p, mirror is one-half of the radius
image distance q from the mirror and focal length f of the mirror. of curvature i.e., f = R/2.
Thus we can write mirror formula as: However, we take the focal
1 1 + 1 .......... (12.1) length of a convex mirror as

f = p q negative. It is because the rays
appear to come from the focal
Equation (12.1) is true for both concave and convex point behind the mirror.
mirrors. However, following sign conventions should be Therefore, for a convex mirror,
f = - R/2.
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